The present invention relates to a light receiving element for use in a rangefinding device, for example in a camera.
There are already known so-called active type rangefinding devices in which light emitted in the form of a spot or a slit from a light emitting element impinges on an object whose distance is to be measured and then the light reflected from the object is focussed on a light receiving element. The light reflected from the object is focussed at a certain position on the light receiving element according to the distance at which the object is located. Therefore, the object distance can be found based on the position at which the reflected light is focussed on the light receiving element.
An example of such prior art active type rangefinding devices is shown in FIG. 1. A light emitting device 10 includes a light emitting element such as a flash lamp 11, a filter 12 for allowing infrared light to pass therethrough, and a projection lens 13. This light emitting device 10 is designed to project infrared light in the form either of a spot or of a thin line toward an object 14 whose distance is to be measured. In the case of a camera, the object 14 is to be photographed when a shutter release button is halfway depressed in a well known manner. A light receiving device 15 includes first and second light receiving units 16A and 16B arranged closely side by side each of which comprises a light receiving element 17 comprising a photoelectric conversion element, a filter 19 for allowing infrared light to pass therethrough, and a detecting lens 20. On a light receiving surface of the light receiving element 17 of either element 17, for example that of the first light receiving unit 16A in this embodiment, there is a masking filter 18 shown in greater detail in FIG. 2 which comprises a light-opaque section 18A having a saw-toothed configuration and a transparent section 18B having a complementarily saw-toothed configuration.
As the light receiving units 16A and 16B are located at predetermined different base lengths from the light emitting device 10, the reflected light from the object 14 is focussed at different positions on the light receiving elements 17 according to the object distance. The light receiving element 17 of the light receiving unit 16A emits an electric output signal A which, because of the provision of the masking filter 18, varies both with the position on the light receiving element at which the reflected light is focussed and with the intensity of the incident light.
By contrast, the light receiving element 17 of the light receiving unit 16B emits an electric output signal B that varies only with the intensity of the incident light and not with the position at which the reflected light is focussed thereon. Accordingly, based on a combination of the electric output signals A and B, for example the ratio of the electric output signals A/B, the subject distance of the object 14 can be obtained. More specifically, because of the fact that the signal ratio A/B depends not on light intensity but only on subject distance, detected signal ratios can be correspondingly converted into subject distances. In the case that such a rangefinding device is used in automatic focussing cameras, a complete depression of a shutter release button causes a focus adjustment of a taking lens according to the signal ratios.
For forming the prior art masking filter 18, a photographic film is used. Specifically, a negative film is exposed to an original pattern having a desired saw-toothed configuration and then developed to form an opaque saw-toothed pattern on the film comprising the masking filter 18. But when a negative film is used for the masking filter 18, it is impossible to have a transparent section 18B with a light transmittance of 100% and an opaque section 18A with a transmittance of zero. Instead, the light opaque and transparent sections 18A and 18B will have transmittances Td and Tc, respectively.
Moreover, in the case of utilizing a photographic film, one problem associated with the masking filter 18 is that, as is shown in FIG. 3, the saw-teeth have rounded tip ends which results from the grain size of the emulsion and the thickness of the emulsion layer on the film.
Due to these drawbacks described above, the prior art masking filter 18 has only an available range T1 of transmittance shown in FIG. 4. This means that light incident on the light receiving element 17 of the light receiving unit 16A can be by no means utilized efficiently over the entire range of 0% to 100%, and this makes it difficult to provide signal ratios A/B accurately corresponding to object distances.